Lifts for wheelchair bound handicapped persons evolved from truck-type tailgate lifts in the mid-60's to early 70's. A wide variety of lift types have been employed, including: chain drive parallelogram lifts of the Deacon U.S. Pat. No. Re. 31,178; chain driven slide tube type lifts, such as The Braun Corporation Lift-A-Way.RTM. lift; rotary lifts such as The Braun Corporation Swing-A-Way.RTM. lift of Braun et al. U.S. Pat. No. 4,664,584; and dual parallelogram arm hydraulic lifts, such as shown in the Savaria U.S. Pat. No. 4,534,450 (original Savaria cafe-door style lift).
The advantage touted for the chain drive lift of the Deacon lift as compared to the dual parallelogram arm hydraulic lift of Savaria is that the chain drive provides a more smooth and less jerky lift motion. Both of these lift types are offered by Ricon Corporation of Pacoima, Calif. as the R30 (Deacon style) and the R2000 (an extensively modified version of the Savaria lift but not covered by the Savaria patent as it has a single platform and dual hydraulic rams).
The model R30 is directed for use in the retail market, i.e., for individual owners of private vehicles, while the R2000 is designed for use in commercial applications, such as in public buses and vans. The reason for the particular use application for each style of lift is due in significant part to the materials and construction used in each lift. In the R30, the arms of the parallelogram linkages are constructed of solid, flat aluminum bars which are not well-suited for withstanding the continuous and varied use demands placed on a commercial lift. The R30 flat bar stock permits too much side sway for use with hydraulics in a commercial environment. In contrast the R2000 uses U-channel steel arms and links which are better suited for operation in a commercial environment.
One disadvantage of the R2000 is that the hydraulic cylinders of the lift are prone to spongy and uneven operation. This results in the dual problems of platform drift and misalignment of the parallelogram arms with respect to each other. Drift occurs when the lift platform is left overnight in the vertical stowed position and it begins to pop partly open under the force of a spring which is provided to power the initial deployment of the lift downward from a near vertical or oververtical position. Drift is symptomatic of wear in the hydraulic seals, which permits fluid to leak past the seals. When this occurs the lift partially descends due to the spring bias and gravity effect. In some instances, this partial opening drift can result in the lift becoming wedged against the inside of the door of the van in which it is mounted. In such a situation it must be first powered closed before the van door can be opened. If the operator does not perceive that drift has occurred, the door and/or lift can be significantly damaged if the operator attempts to open the door.
Further, there is a serious and difficult problem in the synchronization between the two push-type hydraulic cylinders, each of which powers one of the arm linkages. If there is a differential amount of leakage, or air gets in the fluid, as is almost inevitable, or there is differential line loss (friction in the lines), then the two parallelogram linkages descend or lift unevenly. As a result, they do not arrive at the same angular (elevational) position at the same time or at the same rate. This causes a torsional twisting of the lift and places substantial side piston loads on the arm and seal mechanisms of the parallelogram linkages and the hydraulic cylinders. This, of course, increases wear, induces binding and causes elongation of the bushings associated with all the pivot points of the parallelogram linkages. When the binding is suddenly released, the lift has a jerky motion. This uneven and spongy cylinder operation promotes parallelogram arm alignment problems which, in turn, induces wear on all moving parts, including cylinder seals and pivot pins. The uneven wear in turn further compounds the problem of uneven and spongy push-type cylinder operation.
Another aspect of these problems lies in the fact that the lifts must stop in various predetermined positions in the cycle, for example, at the transfer level, that is, where the platform stops at a level even with the van or bus floor to permit ingress into or egress out from the vehicle. In the R2000 two positioning cams are fastened to a hydraulic cylinder cross-pin so that the cams follow the position of the hydraulic cylinder in the cycle. These cams alternately engage and actuate several microswitches associated with the various predetermined positions of the cycle. Since this hydraulic pivot pin is under high stress and experiences excessive side loading from the pushing action of the hydraulic cylinder associated therewith, the bushing surface of this pivot pin tends to become elongated over time. This, in turn, can result in misalignment of the positioning cam. Pivot pin elongation in combination with platform drift significantly upsets the microswitch operation of the positioning cams, thus requiring frequent field readjustments of the positioning cams to restore proper lift operation with precise platform stop positions.
Accordingly, there is a need in the art for improved hydraulic drives for parallelogram arm lifts which have reduced asynchronous movement, jerkiness, platform drift, positioning cam misalignment, and bearing elongation for easier field maintenance and capability of reliable operation in high frequency use transit vehicle environments.